Merge ping6 to ping

There is now a single ping binary, which chooses to use ICMP or ICMPv4
based on the -4 and -6 options, and the format of the address.

Submitted by: Ján Sučan <sucanjan@gmail.com>
Sponsored by: Google LLC (Google Summer of Code 2019)
MFC after: Never
Differential Revision: https://reviews.freebsd.org/D21377